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(#) `contentKey` lambda in AnimatedContent should always use the provided `T` parameter

!!! ERROR: `contentKey` lambda in AnimatedContent should always use the provided `T` parameter
   This is an error.

Id
:   `UnusedTargetStateInContentKeyLambda`
Summary
:   `contentKey` lambda in AnimatedContent should always use the provided `T` parameter
Severity
:   Error
Category
:   Correctness
Platform
:   Any
Vendor
:   Jetpack Compose
Identifier
:   androidx.compose.animation
Feedback
:   https://issuetracker.google.com/issues/new?component=612128
Min
:   Lint 8.0 and 8.1
Compiled
:   Lint 8.7+
Artifact
:   [androidx.compose.animation:animation-android](androidx_compose_animation_animation-android.md.html)
Since
:   1.8.0-alpha01
Affects
:   Kotlin and Java files and test sources
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/animation/animation-lint/src/main/java/androidx/compose/animation/lint/AnimatedContentDetector.kt)
Tests
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/animation/animation-lint/src/test/java/androidx/compose/animation/lint/AnimatedContentDetectorTest.kt)
Copyright Year
:   2023

In `AnimatedContent`, the `contentKey` lambda may be used when the
`targetState` is expected to mutate frequently but not all mutations are
desired to be considered a target state change. So `contentKey` is
expected to always use the given `targetState` parameter to calculate
its result.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/foo/test.kt:13:Error: Target state parameter it is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { if (foo) { /**/ } else { /**/ } }
                 -----------------------------------
src/foo/test.kt:17:Error: Target state parameter it is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { if (foo) { /**/ } else { /**/ } },
                 -----------------------------------
src/foo/test.kt:22:Error: Target state parameter param is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { param -&gt; if (foo) { /**/ } else { /**/ } }
                   -----
src/foo/test.kt:26:Error: Target state parameter param is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { param -&gt; if (foo) { /**/ } else { /**/ } },
                   -----
src/foo/test.kt:31:Error: Target state parameter _ is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { _ -&gt; if (foo) { /**/ } else { /**/ } }
                   -
src/foo/test.kt:35:Error: Target state parameter _ is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { _ -&gt; if (foo) { /**/ } else { /**/ } },
                   -
src/foo/test.kt:39:Error: Target state parameter it is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { if (foo) { /**/ } else { /**/ } }
                 -----------------------------------
src/foo/test.kt:42:Error: Target state parameter it is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { if (foo) { /**/ } else { /**/ } },
                 -----------------------------------
src/foo/test.kt:46:Error: Target state parameter param is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { param -&gt; if (foo) { /**/ } else { /**/ } }
                   -----
src/foo/test.kt:49:Error: Target state parameter param is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { param -&gt; if (foo) { /**/ } else { /**/ } },
                   -----
src/foo/test.kt:53:Error: Target state parameter _ is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { _ -&gt; if (foo) { /**/ } else { /**/ } }
                   -
src/foo/test.kt:56:Error: Target state parameter _ is not used
[UnusedTargetStateInContentKeyLambda]
    contentKey = { _ -&gt; if (foo) { /**/ } else { /**/ } },
                   -
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/foo/test.kt`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
package foo

import androidx.compose.animation.*
import androidx.compose.runtime.*

val foo = false

@Composable
fun Test() {
    AnimatedContent(
        foo,
        contentKey = { if (foo) { /**/ } else { /**/ } }
    ) { if (foo) { /**/ } else { /**/ } }
    AnimatedContent(
        foo,
        contentKey = { if (foo) { /**/ } else { /**/ } },
        content = { if (foo) { /**/ } else { /**/ } }
    )
    AnimatedContent(
        foo,
        contentKey = { param -> if (foo) { /**/ } else { /**/ } }
    ) { param -> if (foo) { /**/ } else { /**/ } }
    AnimatedContent(
        foo,
        contentKey = { param -> if (foo) { /**/ } else { /**/ } },
        content = { param -> if (foo) { /**/ } else { /**/ } }
    )
    AnimatedContent(
        foo,
        contentKey = { _ -> if (foo) { /**/ } else { /**/ } }
    ) { _ -> if (foo) { /**/ } else { /**/ } }
    AnimatedContent(
        foo,
        contentKey = { _ -> if (foo) { /**/ } else { /**/ } },
        content = { _ -> if (foo) { /**/ } else { /**/ } }
    )
    Transition(foo).AnimatedContent(
        contentKey = { if (foo) { /**/ } else { /**/ } }
    ) { if (foo) { /**/ } else { /**/ } }
    Transition(foo).AnimatedContent(
        contentKey = { if (foo) { /**/ } else { /**/ } },
        content = { if (foo) { /**/ } else { /**/ } }
    )
    Transition(foo).AnimatedContent(
        contentKey = { param -> if (foo) { /**/ } else { /**/ } }
    ) { param -> if (foo) { /**/ } else { /**/ } }
    Transition(foo).AnimatedContent(
        contentKey = { param -> if (foo) { /**/ } else { /**/ } },
        content = { param -> if (foo) { /**/ } else { /**/ } }
    )
    Transition(foo).AnimatedContent(
        contentKey = { _ -> if (foo) { /**/ } else { /**/ } }
    ) { _ -> if (foo) { /**/ } else { /**/ } }
    Transition(foo).AnimatedContent(
        contentKey = { _ -> if (foo) { /**/ } else { /**/ } },
        content = { _ -> if (foo) { /**/ } else { /**/ } }
    )
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/animation/animation-lint/src/test/java/androidx/compose/animation/lint/AnimatedContentDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `AnimatedContentDetector.unreferencedParameters`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=612128.

(##) Including

!!!
   This is not a built-in check. To include it, add the below dependency
   to your project.

```
// build.gradle.kts
implementation("androidx.compose.animation:animation-android:1.8.0-alpha07")

// build.gradle
implementation 'androidx.compose.animation:animation-android:1.8.0-alpha07'

// build.gradle.kts with version catalogs:
implementation(libs.animation.android)

# libs.versions.toml
[versions]
animation-android = "1.8.0-alpha07"
[libraries]
# For clarity and text wrapping purposes the following declaration is
# shown split up across lines, but in TOML it needs to be on a single
# line (see https://github.com/toml-lang/toml/issues/516) so adjust
# when pasting into libs.versions.toml:
animation-android = {
    module = "androidx.compose.animation:animation-android",
    version.ref = "animation-android"
}
```

1.8.0-alpha07 is the version this documentation was generated from;
there may be newer versions available.

NOTE: These lint checks are **also** made available separate from the main library.
You can also use `androidx.compose.animation:animation-lint:1.8.0-alpha07`.


[Additional details about androidx.compose.animation:animation-android](androidx_compose_animation_animation-android.md.html).
(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("UnusedTargetStateInContentKeyLambda")
  fun method() {
     AnimatedContent(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("UnusedTargetStateInContentKeyLambda")
  void method() {
     AnimatedContent(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection UnusedTargetStateInContentKeyLambda
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="UnusedTargetStateInContentKeyLambda" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'UnusedTargetStateInContentKeyLambda'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore UnusedTargetStateInContentKeyLambda ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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